1. Field of the Invention
The present invention relates to an optical waveguide-forming paste composition. More specifically, the present invention relates to an optical waveguide-forming paste composition including an organic material and barium sulfate particles dispersed therein and to an optical waveguide using the composition.
2. Description of the Related Art
Paste compositions including organic materials such as resins and inorganic particles dispersed therein and cured materials obtained by curing such paste compositions are widely used for printing paints, lubricants, cosmetics, adhesives, release agents, components of displays or circuit boards, and so on. The purposes of dispersing inorganic particles in resins include imparting excellent properties such as thermo-mechanical properties, electromagnetic properties, and optical properties, which are not achievable only with resins, and reducing the content of resins and making the manufacturing cost low in a case where the resins used are expensive. In recent years, there have been attempts to use very small inorganic particles with particle diameters of several tens to several nanometers for dispersions for the purpose of improving the surface flatness or transparency of materials or meeting requirements for micro-machining of semiconductors or the like in various technical fields. Particularly in the field of optical interconnection technology, there have been investigations to develop techniques for dispersing inorganic particles with particle diameters of several tens to several nanometers into resins so that temperature-dependent changes in the refractive index or the dimension of optical waveguide materials can be suppressed, while the transparency of the materials can be ensured.
A method for dispersing inorganic particles into a resin includes first preparing a dispersion solution containing an organic solvent and inorganic particles well dispersed therein and then mixing the dispersion solution with a resin. Commercially available inorganic particles with a mean particle diameter of several tens to several nanometers are often provided in the form of agglomerates (secondary particles) with a mean particle diameter of several tens μm in which individual particles (primary particles) are moderately agglomerated. In order to prepare a dispersion solution of inorganic particles with a mean particle diameter of several tens to several nanometers, therefore, the secondary particles should be deagglomerated in an organic solvent and a dispersion solution containing stably dispersed primary particles should be prepared. However, in such a case where primary particles have a mean particle diameter of less than 50 nm, once dispersed particles may be frequently agglomerated to make a further dispersion process difficult in many cases, because the ratio of the surface area to the volume of the particles is significantly high.
Therefore, there is disclosed a method that includes adding an organic material having a certain functional group or the like at its end, called a dispersant, so that the functional group of the dispersant is located on the surface of the inorganic particles to inhibit the contact between the inorganic particles, which can suppress reagglomeration of primary particles and improve the dispersibility. As an example of this technique, there is proposed a nickel colloid solution containing nickel colloid particles with a mean particle diameter of 20 to 40 nm, a non-polar high-molecular pigment dispersant, and an organic solvent (see for example Patent Literature 1). There is also disclosed a method of using a dispersant to disperse barium sulfate particles into an organic solvent (see for example Patent Literature 2).
However, when a cured material is produced by a method including mixing a resin and a dispersion solution of inorganic particles prepared with conventionally known dispersants to form a paste composition and then curing the paste composition to form a cured material in which the inorganic particles are dispersed, the curing reaction of the resin by photoirradiation or heating is more likely to be insufficient, than when no inorganic particles are used. Therefore, the resulting cured material may have low thermo-mechanical properties. In addition, when photolithographic patterning is performed on a film produced with a paste composition containing inorganic particles and an optically curable resin, the exposed area may be insufficiently cured so that the composition may be eluted during development to form an unclearly patterned portion, or the non-exposed area may have low solubility so that a paste composition residue may be produced during development. Therefore, photolithographically produced optical waveguides may have large optical propagation loss.
It is also known that addition of a polymerizable group-containing phosphoric ester monomer in the process of curing a resin can produce an advantageous effect such as an improvement in the antistatic performance or flame retardancy of the cured material or an improvement in the dispersibility of pigments (particles) dispersed in the resin (see Patent Literature 3).
In general, the dispersibility of pigments (particles) significantly depends on whether the pigment used is an organic or inorganic material. Even among the inorganic pigments, however, for example, barium sulfate particles significantly differ in surface potential, surface smoothness or the like from alumina particles, and, therefore, a certain additive effective in improving the dispersibility of one may reduce the dispersibility of the other. Patent Literature 3 only discloses that inorganic pigments may be used and does not specifically disclose the type of inorganic pigments that can influence the dispersibility as mentioned above.    Patent Literature 1: Japanese Patent Application Laid-Open (JP-A) No. 2004-124237 (Claims and Examples)    Patent Literature 2: JP-A No. 2006-106708 (page 18)    Patent Literature 3: JP-A No. 2003-146992 (page 2)